This invention relates to producing ion beams suitable for ion implantation and to improved ion implantation apparatus and techniques.
Ion implantation is useful for producing near surface modification of materials, but presently the principal commercial application of ion implantation is in the manufacture of large scale integrated (LSI) circuits. Generally, ions of a selected species are implanted into the surface regions of a semiconductor wafer to alter the electrical characteristics of the wafer. The depth of implantation is determined by the energy of the ions, while the throughput of the implantation process is determined by the ion beam current. As the lateral dimensions of semiconductor devices become smaller, the vertical dimensions must also be reduced. For example, in the manufacture of devices with minimum dimensions of 0.25 microns or less, certain process steps (e.g., in the creation of the source and drain regions of field effect devices) require ion beam energies as low as 1-5 key and typical ion doses of about 5.times.10.sup.15 ions/cm.sup.2.
Space-charge forces within an ion beam, however, tend to limit the beam current levels achievable at low ion beam energies. These forces cause the ions in the beam to diverge outward, producing an unmanageably large beam envelope. Space-charge blow-up is the rate at which the transverse dimension of the beam increases with distance along the beam axis. This is proportional to a mass-normalized beam perveance (.epsilon.) EQU .epsilon.=IM.sup.1/2 E.sup.-3/2 ( 1)
where I is the beam current, M is the ion mass, and E is the ion energy. For a typical ion implanter configuration, space-charge effects become limiting at a perveance of .epsilon..about.0.2 (mA)(amu).sup.1/2 (keV).sup.-3/2. Thus, an 80 keV arsenic beam becomes space-charge limited at a current of about 1.7 mA, while a 5 keV beam is space-charge limited at a current of only about 0.03 mA.
Referring to FIGS. 13 and 13A, space-charge effects tend to limit the performance of a typical state-of-the-art ion implanter at low beam energies. When the beam energy drops below about 10 keV, the beam current transmitted through such an ion implantation system is rapidly reduced because of beam blow-up. This significantly effects the throughput of the implanter. As shown in FIG. 13A, for a given ion dose greater than about 10.sup.14 ions/cm.sup.2, selected to achieve desired electrical characteristics in an implanted wafer, the throughput drops drastically as the required dose increases for a 1 mA beam relative to a 10 mA beam.
Another consideration in the design of ion implantation equipment is surface charging of the workpiece. When a semiconductor wafer bearing one or more insulating layers passes through an ion beam, the wafer surface will become charged. Such wafers can experience dielectric breakdown when wafer charging occurs. Also, as a wafer becomes, e.g., positively charged, electrons are swept from the ion beam, increasing the space-charge forces within the beam.